Pneumatic and electric viscous fluid dispensers have been developed for dispensing applications requiring precise placement of a viscous fluid. Pneumatic dispensers have a significant advantage in that the pneumatic solenoid operating the dispensing valve can be made very strong, so that the dispensing valve operation is essentially independent of the viscosity of the fluid being dispensed. However, pneumatic dispensers have disadvantages in that they generally have a shorter life than electric fluid dispensers, and the operation of the pneumatic solenoid is subject to less precise control than the electric solenoid in an electric fluid dispenser. Therefore, in some applications, electrically operated viscous fluid dispensers are preferred over pneumatic viscous fluid dispensers.
Generally, electrically operated dispensers include an electromagnetic coil surrounding an armature that is energized to produce an electromagnetic field with respect to a magnetic pole. The electromagnetic field is selectively controlled to open and close a dispensing valve by moving a valve stem connected to the armature. More specifically, the forces of magnetic attraction between the armature and the magnetic pole move the armature and valve toward the pole, thereby opening the dispensing valve. At the end of a dispensing cycle, the electromagnet is de-energized, and a return spring returns the armature and valve stem to their original positions, thereby closing the dispensing valve.
In the operation of an electric viscous fluid dispensing gun, the coupling between the coil and the armature is not efficient; and therefore, in order to achieve the highest actuation speed, a current pulse or spike is typically provided to the coil during an initial turn-on period in order to initiate the motion of the armature as quickly as possible. However, maintaining such a level of current to the coil quickly and substantially increases coil temperature. Further, maintaining such a high level of current increases the time required for the energy stored in the coil's inductance to dissipate, thereby increasing the turn-off time and the time required to close the fluid dispenser. Therefore, after the initial current spike, the current through the coil is normally reduced to approximately the minimum value required to hold the armature in its open position by overcoming the opposing force of the return spring. Such a stepped current waveform is useful in reducing the current induced heat load in the coil, thereby allowing the coil to operate at a lower temperature than if the stepped waveform were not used. However, as is described below, the operation of the coil and armature during the fluid dispensing process creates other heat related issues that impact the quality of the fluid dispensing process.
The continued development and use of viscous fluid electric dispensers has resulted in more demanding performance specifications as well as a greater understanding of how heat in the dispenser can potentially effect performance. For example, the electric coil of an electric dispensing valve normally is not capable of providing the same forces as a pneumatic solenoid and therefore, is more subject to changes in resistance to valve stem motion that may be caused by changes in viscosity of the fluid being dispensed. Thus, as the viscosity of the fluid being dispensed changes, the load on the electromagnetic coil changes, and the time required to open and close the dispensing valve will likewise change. Such changes in timing of the dispensing valve opening and closing will change the location of the adhesive being dispensed on the substrate.
In addition to the above, newer applications have more demanding performance specifications and require ever-increasing gun speeds, that is, a shortening of the time required to open and close the dispensing valve. The operational speed of the dispensing valve can be increased by increasing the electrical power applied to the electric coil operating the valve. The electrical power is normally increased by increasing the current being supplied to the coil which also adds heat to the coil, thereby causing the temperature of the coil to rise. A hotter or higher coil temperature impacts the consistency of the viscous fluid dispensing in several ways. First, heat from the coil is conducted through the armature and the valve stem which is adjacent the valve seat and is surrounded by the viscous fluid. As the temperature of the armature fluctuates, for example, goes up, the viscosity of the fluid to be dispensed likewise fluctuates and, in this example, decreases, thereby changing the flow of the viscous fluid from the dispenser.
Second, the speed at which the armature can be moved between the open and closed positions is a function of the rate of change of current in the coil, which, in turn, is controlled by the electrical time constant of the coil. The electrical time constant is a function of the coil resistance which, in turn, is a function of temperature. The coil utilized in the viscous fluid dispenser discussed herein can experience an approximately 50% variation in resistance over its normal range of operating temperature. Such a change in resistance substantially affects the electrical time constant of the coil, thereby similarly affecting the speed at which the coil can open and close the valve.
The thermal time constant of the coil is a function of the coil mass and its thermal connections to surrounding materials such as the gun body and ambient temperature. The thermal time constant of the coil and its surrounding thermal system affects the time required for the thermal system to reach a steady state condition. When the dispensing system is running at a constant speed, and a steady state condition is achieved, the thermal time constant normally does not present a source of variation in the operation of the dispensing coil. However, the steady state condition can change for several reasons, for example, if the production line speed is either increased or decreased or, the dispensing gun is not operating and in the standby mode. Either of those conditions causes the coil temperature to change, and the thermal time constant presents a source of variations in the operation of the viscous fluid dispenser.
Of further concern is the maximum temperature rating of the coil wire insulation. Under normal operating conditions, the temperature rating of the wire insulation exceeds the wire temperature. However, in a worse case situation, if the temperature of the wire exceeds the temperature rating of the wire insulation, the integrity of the coil wire insulation may be compromised, thereby causing coil windings to short-circuit together. Any coil windings that short-circuit together will change the resistance of the coil and potentially adversely effect the consistency of the fluid dispensing operation of the dispenser.
Thus, by using a stepped current waveform, known electric fluid dispensers attempt to reduce the temperature of the coil. Further, it is known to utilize a heater in a manifold to which the fluid dispenser is mounted to control the temperature of the fluid circulating through the manifold and the fluid dispenser, thereby indirectly controlling the temperature of the dispenser itself. However, as will be appreciated, there have been no attempts to control the temperature of the fluid dispenser directly with a self contained device in order to maintain the electric fluid dispenser at a constant temperature.